I have a table in a Rails application which (in schema.rb) looks like: create_table "users", :force => true do |t| t.string "name", :null=>false t.string "address", :null=>false end I would like to write a rails migration to allow nulls for the address field. i.e. after the migration the table looks like this: create_table "users", :force => true do |t| t.string "name", :null=>false t.string "addr